The invention generally relates to an ultrasonic device, and in particular the invention relates to an ultrasonic cavition device having a wire which is composed of a metal having a relatively low modulus of elasticity such as titanium. The wire has a length and a diameter dimensioned for specific applications of removal of unwanted material. One application, for example, is the removal of plaque from a human artery by the action of fluid cavitation. Another application, for example, is the removal of contaminants from otherwise inaccessible areas of industrial parts, such as aerospace parts.
Related U.S. Pat. Nos. include:
3,352,303, issued Nov. 14, 1967, PA1 3,565,062, issued Feb. 23, 1971, PA1 3,589,363, issued Jun. 29, 1971 PA1 3,618,594, issued Nov. 9, 1971, PA1 3,861,391, issued Jan. 21, 1975, PA1 4,474,180, issued Oct. 2, 1984, and PA1 4,660,573, issued Apr. 28, 1987.
Related publications include:
(1) "A Critical Appraisal of Methods for Disruption and Extraction of Urinary Calculi Especially with Ultrasound", by Harold Lamport and Herbert F. Newman, The Yale Journal of Biology and Medicine, Volume 27, Number 5, Jun., 1955. PA0 "A New Method to Cure Thrombi by Ultrasonic Cavitation ", by U. Stumpff, R. Pohlman and G. Trubestein, Ultrasonics International 1975 Conference Proceedings. PA0 (3) "Preliminary Feasibility Studies Using an Ultrasonic Device for Endarterectomy", by Harry L. Finkelstein, M.D., et al, The Mount Sinai Journal of Medicine, Volume 46, Number 2, Mar. -Apr., 1979. PA0 (4) Technical Support Package Entitled "Speculation on Ultrasonic Disintegration of Arterial Deposits", George C. Marshall Space Flight Center, Marshall Space Flight Center, Ala. 35812, Winter 1983, Volume 8, Number 2, MFS-25161.
The prior art devices which have endeavored to remove unwanted material from the human body or other inaccessible areas have severe limitations. In most of these devices the sonic wave guide or active probe is short and rigid. In the case of ureter stones, the active tube or wire is about 45 cm long (18"), but is still fairly rigid. The urinary duct is straightened by the metal wire insertion. This cannot be the case, however, if one is to reach the coronary arteries percutaneously by entering a blood vessel at the neck or armpit. Similarly, it is not possible to thread rigid wire down the leg when the artery or vein to be cleaned is sinuous. In these instances, the ultrasonic waveguide or wire must be long, narrow and highly flexible in order to pass through the vessel without causing damage. Further, because there is a standing soundwave in the wire, the metal composition must provide minimum attenuation of the sonic energy. In layman's terms, the metal must be capable of ringing like a bell. Experiments have shown that this property is especially important when the wire is bent, since bending greatly increases the tendency for energy in the sound wave to be dissipated as heat and a significant temperature rise would not be tolerable.
The process of transmitting sound waves into arteries of the heart or leg by inserting a wire through the blood vessels was first tried in the early 1960's. At that time standard ultrasonic instruments became available for laboratory homogenizing and emulsifying. These devices were portable ultrasonic probes, about 1/2" diameter, and had about 200 times the intensity or vibration amplitude of the more common ultrasonic tank cleaners. A long wire could be attached to the vibrating end of these probes, and the wire then maneuvered through the blood vessels. Energizing the probe would in turn activate the wire. Although reaching occlusions in the leg by wire was sometimes successful, the process of plaque removal was not. Energy transmitted to the vibrating tip was insufficient to produce the necessary cavitation (bubble collapse) for disrupting or liquefying plaque. Removal of simple blood clots or thrombi in the leg, a far less demanding application, was also unsuccessful. For this and other reasons, the device disclosed in U.S. Pat. No. 3,352,303, 1967, "Method For Blood Clot Lysis" had significant limitations. Arterial plaque could be removed by ultrasound as discussed in publication number 3 cited above, but the procedure was performed with a relatively short rigid probe and it was necessary to completely open the artery and expose the occlusive plaque.
The idea of threading an ultrasonic wire into blood vessels has been tried many times in the last 25 years, and with good reason. Arteriosclerosis is the number one killer in the western world. Unfortunately, a vibrating percutaneous wire system never showed promise in treatment of this disease, nor did it provide data suitable for reporting in the scientific literature. When power was increased to the wire, it overheated, and moreover, tended to vibrate itself apart. Well before reasonable displacement was achieved at the tip, the wire broke, usually at the attachment point to the horn.
In publication 4 cited above, the authors suggest that removing arterial deposits by ultrasonic disintegration would be an excellent idea. After a number of investigations, they concluded that this area of ultrasound was apparently virtually unexplored.
U.S. Pat. No. 4,474,180, 1984, Apparatus for Disintegrating Kidney Stones, attempts to increase the useful life of wire in kidney stone breaking by utilizing a long damper tube. This tube closely fits around the wire near its attachment to the horn, and reduces undesirable transverse vibration. However, a close fitting tube placed anywhere along the wire would limit both longitudinal as well as lateral motion, and therefore the tip displacement would be much reduced. A separate catheter referred to in the same patent, is specifically made loose to avoid such a difficulty. In U.S. Pat. No. 3,861,391, 1975, Apparatus for Disintegration of Urinary Calculi, no mention is made, or suggestions offered, as to the type of metal to be used in this patent or the previously cited patent. It was not important to the inventor. The same is the case for U.S. Pat. No. 4,660,573, 1987, Ultrasonic Lithotriptor Probe.
Because of its great strength and high proportional limit, stainless steel and other high strength steels were used as wire or waveguides in past blood clot disruption experiments. These metals are now in standard use as kidney stone breakers. In U.S. Pat. No. 3,565,062, 1971, Ultrasonic Method and Apparatus for Removing Cholesterol and Other Deposits from Blood Vessels and the like, the inventor goes into detail as to how and why his device works. He suggests the use of Monel and stainless steel as waveguides. Similarly in U.S. Pat. No. 3,618,594, 1971, Ultrasonic Apparatus for Retinal Reattachment the inventor suggests the use of Monel for the active probe. Similarly in U.S. Pat. No. 3,352,303, cited above, the inventor suggests stainless steel or Monel. Also in U.S. Pat. No. 3,589,363, the vibration transmitting material is suggested to be Monel metal with only the operative tip comprising "an extremely hard, sterilizable material, such as titanium".
One problem with certain prior art ultrasonic device is that the wire, which is relatively short and relatively rigid, is not suitable for entering into an artery in the armpit or neck area of the human body, and for passing through the artery to the vicinity of the heart.
In one ultrasonic kidney stone device, U.S. Pat. No. 3,861,391, cited above, the wire is specifically designed to increase whipping motion at the tip so as to break the stones faster. Kidney stones however, for the most part are not broken by cavitation, or the making and breaking of bubbles, but by pounding or a jackhammer effect.
In U.S. Pat. No. 3,565,062, cited above, the inventor recognizes that heat generation is a problem during blood clot removal and indicates that discrete points on the wire waveguide will glow red hot. The inventor "solves" the problem by constantly changing the frequency so that nodes and anti-nodes on the wire physically shift, as well. But unfortunately, the wire, for the most part, will only resonate at the fundamental frequency for which it was designed, and changing frequencies is tantamount to turning the apparatus on and off.
In publication 2 cited above, the authors discuss dissolving thrombi utilizing an ultrasonic waveguide but apparently if no liquid is supplied via the waveguide, the temperature becomes so high that the wall of the vessel can be burned.
Another problem encountered in the perecutaneous removal of plaque, or other applications of this kind, is the attachment of the wire to the horn. Since the horn and wire cannot be fabricated as one continuous piece, energy losses will occur at this junction. A similar problem arises with devices for kidney stone breaking. Harmonic generation and reflections occur at the attachment site, and these attenuate the fundamental wave and cause power loss. When the necessary amount of power is finally delivered to the wire tip, the attachment point will usually heat up and break in a few seconds unless cooling water is applied. This is the case with both kidney stone breaking and cataract removal, but would be even more noticeable in the plaque removal process where a small diameter and longer wire is necessary and the amplitudes required are higher.
Methods of attaching ultrasonic wire and tubes in the past include: brazing, welding, threading, epoxying, and clamping. These were found inadequate in present plaque and blood clot application because, except for threading, each causes too much power loss or heating. Perhaps 90% of the applied energy is wasted in kidney stone breaking, although the high strength metal used as transmitting wire or tubes can withstand the extra stress and heat. Furthermore, it is not difficult to cool the vibrating wire with running water, or saline, as a kidney stone is being sonicated.
Brazing or soldering makes a poor bond with titanium. Brazing, too, tends to anneal stainless steel so that some of the sound wave is damped or reflected. Welding temperatures required for titanium, change the grain structure at the heated junction, producing non-linearity as well as weakening the metal. Both epoxy bonding, and attachment by clamping or thumb screw, result in too much loss and heat. Producing a screw thread on titanium wire 1 mm in diameter is not possible at present.
In U.S. Pat. No. 4,474,180, cited above, an improved method of attaching wire to a vibrating ultrasonic horn is described. The wire "life" is prolonged to a little over a minute, rather than the previous time to breakage of 20 seconds. This performance is clearly not acceptable in our projected blood vessel applications where much longer times will likely be needed to destroy arterial plaque. Moreover, just the threat of wire breaking during insertion into the heart arteries is unacceptable.